1. Field of the Invention
The present invention generally relates to monitoring the motion of a scan mirror employed for sweeping a light beam in electro-optical readers for reading indicia such as bar code symbols, or in image projectors for displaying images and, more particularly, to a motor drive circuit with reduced coil crosstalk in a feedback signal indicative of such mirror motion.
2. Description of the Related Art
Electro-optical readers are well known in the art for electro-optically transforming a spatial pattern of graphic indicia, known as a symbol, into a time-varying electrical signal which is then decoded into data. Typically, a light beam generated from a light source is focused by a lens along an optical path toward a target that includes the symbol. The light beam is repetitively swept along a scan line or a series of scan lines arranged in a raster pattern over the symbol by moving a scan mirror located in the optical path. A photodetector detects light scattered or reflected from the symbol and generates an analog electrical signal. Electronic circuitry converts the analog signal into a digitized signal having pulse widths corresponding to physical widths of bars and spaces comprising the symbol, and a decoder decodes the digitized signal into data descriptive of the symbol.
The repetitive sweeping of the light beam is performed by a drive, typically a motor having a rotor oscillatable about an axis. A permanent magnet and the scan mirror are jointly oscillatable with the rotor. The motor is driven by a drive coil wound on a bobbin that is located physically close to the permanent magnet. A feedback coil is also wound on the same bobbin. In response to an alternating voltage drive signal applied to the drive coil, the electromagnetic field produced by the drive coil interacts with the permanent magnetic field of the magnet, thereby jointly moving the magnet and the mirror. The frequency of the drive signal in the drive coil is the same as the rotor motion, with one cycle of the drive signal corresponding to one cycle of rotor motion. The amplitude of the drive signal in the drive coil is proportional to the velocity of the rotor motion. The polarity of the drive signal in the drive coil is dependent on the direction of rotor motion such that a positive half cycle of the drive signal indicates that the rotor is moving in one drive direction, and a negative half cycle indicates that the rotor is moving in the opposite drive direction. Zero crossings of the drive signal occur when the rotor reaches its maximum travel at each end of a respective scan line. At each zero crossing, the rotor stops for an instant and reverses drive direction.
The feedback coil is useful for a variety of purposes. It generates an alternating voltage signal, known as a feedback signal, due to the movement of the magnet. The frequency and polarity of the feedback signal generated in the feedback coil corresponds to the frequency and polarity of the drive signal. An electrical drive monitoring circuit is often employed to monitor the amplitude of the feedback signal and, for example, turn the light source off if the amplitude falls below a predetermined threshold, thereby indicating that the drive is malfunctioning. An electrical closed loop control circuit is also often employed to process the feedback signal to make decisions about how to continue driving the motor. Still another electronic circuit that is often employed processes the zero crossings of the feedback signal to derive a start-of-scan (SOS) signal that represents rotor motion and is used to synchronize the scan lines.
Although generally satisfactory for its intended purpose, the use of the feedback coil for monitoring for drive failure, for driving the drive motor, and for generating the SOS signal causes problems. There is undesirable magnetic coupling or crosstalk between the drive and feedback coils. To remove such unwanted coupled signals and the resulting noise and distortion, electronics are usually added to actively cancel and filter the coupled signals, and filtering is necessary to ensure control loop stability. Without such filtering, the loop would oscillate based on feedback through the coils, thereby impairing motor malfunction detection. The filtering also changes the frequency of the electromechanical system, thereby causing the motor to oscillate off its natural or resonant frequency, and thereby impairing the drive efficiency. Since filtering introduces phase delays, the SOS signal will never represent the true position of a beam spot of the scanning light beam relative to the leading bars and spaces in a target symbol. This problem is solved in the art by adding and adjusting electronics to advance or delay the SOS signal depending on the type of motor used. The art has also proposed the use of optical feedback circuits. In addition, when the feedback coil is coupled to the drive coil, an annoying buzzing sound is sometimes generated.
Another arrangement, other than a symbol reader, that repetitively scans a light beam in a raster pattern over a target is an image projector for projecting an image on a display surface, for example, a screen. Typically, one or more energizeable lasers of different wavelengths project respective laser beams toward the screen, while an oscillating drive sweeps the beams in scan lines over the screen. Usually, a pair of scan mirrors is employed to sweep the beams in mutually orthogonal directions. The lasers are energized and deenergized during each sweep to create a bit-mapped image on the screen for viewing. As in the case of readers, at least one of these scan mirrors is oscillated by a drive which includes a motor having feedback and drive coils, as described above, with their attendant problems of cross-coupled signals, extra hardware, phase delays and annoying sounds. Crosstalk is a more severe problem in image projectors, because the motion or velocity of the scan mirror and, hence, of each scan line swept by the scan mirror must be very highly controlled to be a constant value for both right-to-left and left-to-right scan lines. Otherwise, the projected image will be degraded.